1. Field of the Invention
The present invention relates to a laser drive device, a method of controlling the same, and a storage medium, and more particularly to a laser drive device used for an exposure device of an image forming apparatus, such as a printer.
2. Description of the Related Art
In an image forming apparatus or the like using an electrophotographic process, a laser diode is generally used as a light emitting element so as to convert electric signal pulses to optical pulses. Further, the laser diode is required to emit a desired amount of light even when the temperature of the laser diode changes.
However, light emission characteristics of the laser diode are largely dependent on the temperature of the laser diode, and a threshold current varies with changes in the temperature of the laser diode. In the laser diode, a light emitting delay occurs as a phenomenon specific to the laser diode. To improve emission responsiveness of the laser diode during light emission, a bias current is supplied to the laser diode when light is not emitted during image formation. Further, to improve the emission responsiveness, it is preferable that the bias current as close to the threshold current as possible is supplied to the laser diode.
However, as mentioned above, the threshold current is largely dependent on the temperature of the laser diode and is different between individual laser diodes, and hence if the bias current supplied to the laser diode is set to a fixed value, the emission responsiveness can be lowered, and further, a laser beam may be emitted with such a light amount as will change the surface potential of a photosensitive member.
By the way, there has been proposed a laser drive device for use in an image forming apparatus, which is configured to determine a bias current supplied to a laser diode according to two light amounts of laser light emission and drive currents corresponding to the light amounts (bias APC (Automatic Power Control): see e.g. Japanese Patent Laid-Open Publication No. H11-245444).
If the conventional bias current control method is used in an apparatus that exposes a photosensitive member with a plurality of laser beams emitted from a plurality of light emitting elements so as to increase the speed of image formation, there occurs a problem that the bias current cannot be set with high accuracy. When determining a value of the bias current, drive currents are supplied to the light emitting elements which are objects to be controlled, so as to cause the light emitting elements to emit respective two amounts of light: a first light amount P1 (APC_H) and a second light amount P2 (APC_L).
In doing this, the drive current is controlled while detecting the light amount using a detection device, such as a PD (photodiode), and there are sampled a drive current flowing when the first light amount is detected and a drive current flowing when the second light amount is detected. Based on these results, light emission characteristics in a laser oscillation region of the light emitting elements to be controlled are determined. Then, a threshold current is calculated based on the light emission characteristics, and the bias current is determined based on the threshold current.
When one light emitting element is under APC, the bias current continues to be supplied to the other light emitting elements than the one light emitting element under APC, so as to secure the emission responsiveness thereof. The above-mentioned PD is commonly used for the plurality of light emitting elements, and is arranged in the vicinity of the plurality of light emitting elements. Therefore, the supply of the bias current causes spontaneous emission of laser beams from the light emitting elements, and the spontaneously emitted laser beams enter the PD. Therefore, the detection result from the PD includes light amounts of laser beams emitted from the other light emitting elements than the one under APC.
When determining the threshold current based on the two light amounts and the drive currents corresponding to these light amounts as mentioned above, it is necessary, for accurate calculation of the threshold current, that one of the two light amounts (hereinafter referred to as APC_H) is set to a target light emission amount and the other (hereinafter referred to as APC_L) is set to a light amount as low as possible.
FIG. 13 is a diagram showing characteristics of the laser diode of the conventional laser drive device.
Referring to FIG. 13, the characteristics of the laser diode (relationship between the drive current and the light amount) are indicated by a curve A, and the threshold current is represented by Ith. In this case, when the laser diode is influenced by bias light emission of the other light emitting element(s), the threshold current deviates from Ith by ΔIth, and the characteristic curve indicative of the characteristics of the laser diode is changed to a curve B.